J. E. Oliver, The Philosophical Review (1892)
“ For instance, shall we think of the quasi-perpendicularity as mutual; the physical forces exerting a merely directive effect upon the spiritual phenomena, as well as the spiritual forces upon the physical phenomena, and the law of conservation presumably holding in the spiritual as well as in the material system? Is there any spiritual force that involves the element of Time, or Inertia, and in virtue of which every system of spiritual forces must be in equilibrio just as a falling stone hangs balanced between the pull of gravity and its own resistance to further acceleration? ”
